This invention relates to a compact heat exchanger and/or fluid mixing means which incorporates a series of plates having apertures which define a plurality of passages through which fluid may flow.
Compact heat exchangers are characterised by their high “area density” which means that they have a high ratio of heat transfer surface to heat exchanger volume. Area density is typically greater than 300 m2/m3 . and may be more than 700 m2/m3. Such heat exchangers are typically used to cool (or heat) process fluids.
One well known but expensive to manufacture type of heat exchanger is the so-called tube and shell heat exchanger. Essentially such heat exchangers consist of an exterior tubular shell through which run a number of longitudinally-extending smaller diameter tubes carrying one or more fluids. Other fluids, with which heat is to be exchanged, typically pass transversely across the heat exchanger such that heat is exchanged through the tube walls. A large number of tubes may be needed and they each have to be individually and accurately fixed/secured into a header plate at each end of the shell. In each case holes need to be drilled in the header plates very accurately to locate the tubes. High quality tested tubing then needs to be assembled into the plates and brazed or welded or mechanically-expanded into position. As the tubes are reduced in diameter to increase surfaces available for heat transfer and hence performance/compactness, the more difficult and expensive such configurations become to manufacture.
A second known type of heat exchanger is the so-called primary plate/secondary plate type exchanger in which a stack of plates is assembled, the stack having primary plates which directly separate two different fluid streams and secondary plates between adjacent primary plates. The secondary plates act as fins which add to the strength of structure and may be provided with perforations to provide additional flow paths for the fluids. The plates are usually bonded together by brazing but this may have the disadvantage of affecting the physical properties of the plates in the brazed regions—or may introduce into the system, by means of the braze material, a potentially less satisfactory structure in terms of strength and corrosion resistance. It has been proposed to bond the plates together by diffusion bonding but a satisfactory construction that can withstand the high pressures involved has not been achieved and the fins may buckle during the bonding process.